Mr Murdoch said today that Ms Pulver was seated but had been trapped in an awkward position for the time it took to remove the device.

"She's been kept in a very uncomfortable position in excess of 10 hours. She will be a little uncomfortable for some time."

Police said Ms Pulver did not know the person who came into the house and placed the device.

Madeleine's father, William Pulver, waits at the scene last night with shocked family and friends. Photo: Brendan Esposito

During the 10-hour ordeal, bomb technicians X-rayed the device but the initial scans could not determine what it was.

Mr Murdoch said that, as the ordeal continued, Ms Pulver remained in good spirits but that it was getting cold in the front room of the home in which she was trapped.

Ms Pulver's parents had no idea why their daughter had been targeted, he said.

"They were clearly and understandably very upset by what had happened.

"They were very, very concerned for their daughter as any parent would be.

"They were at a loss to explain why them, why they'd been ... why this had happened to them as opposed to anyone else."

NSW Police sought advice from British military experts and the Australian Federal Police as they worked to make the device safe.

Ms Pulver was taken to the Royal North Shore Hospital for checks and was released at 3am today.

Police searching

This morning, six officers from the riot squad walked from barriers, which have been erected 50 metres from the home, in a straight line down towards the house.

They searched a traffic island in the middle of Burrawong Avenue about 11.30am.

They also looked under the cars parked outside the house and searched a skip right in front of the house.Riot squad officers were also searching flower beds of neighbouring houses.

'Too early to establish motive'

Mr Murdoch told reporters outside the house that it was "far too early" to establish a motive for the attack.

A police media spokeswoman would not confirm it was an extortion attempt.

After Ms Pulver's family contacted police, saying their daughter needed help, a general duties police car with two officers went to the scene.

"What they saw was a very distressed young lady with what we believed to be at the time an improvised explosive device attached to her body," Mr Murdoch told 2UE radio.

Nearby residents were immediately evacuated and bomb technicians, negotiators and detectives were summoned.

Mr Murdoch praised the courage and calmness of a young policewoman who was first on the scene.

"In the initial car crew that attended there was a young female officer on board who took it upon herself to, while her mate was off summoning all these other resources and evacuating the neighbours, to stay with her through about the first two hours of her ordeal," he said.

"She was not wearing any protective clothing or equipment; she wasn't trained as a negotiator, but she made the decision herself, this young officer, to stay with Madeleine and make sure she tried to remain calm and she wasn't left alone; [she] provided moral support for her and she did an outstanding job in that regard."

A note left with the young woman was coherent and articulate, leaving police in no doubt that the attacker knew what he was doing, Mr Murdoch said.

He said it remained a mystery why the Pulver family had been targeted, but it did not appear random.

"I hardly think that the elaborate nature and sophistication of this device was the result of someone who picked on someone randomly," he said.

The family was completely at a loss to explain why they had been targeted.

"The offender went to a lot of trouble for a particular reason, but what that reason was, police are still working to determine," Mr Murdoch said.

The man had been disguised when he put the device around Ms Pulver's neck, he said, but he declined to give any details of the disguise.

Neighbours and friends react

Rob Waterhouse, bookmaker and the husband of racehorse trainer Gai Waterhouse, described the Pulvers as friends of friends and said outside his Burrawong Avenue house this morning that the bomb threat was "unbelievable".

"I think it's just one of those one-off things," Mr Waterhouse said.

A neighbour who was walking his dog near the house this morning said: "If it can happen here it can happen anywhere."

One of Madeleine’s school friends, Jack Holliday, 19, brought a bunch of pink tulips to the house with a message on a card saying: ‘‘Thinking of you in a difficult time, Maddie’’.

Holliday said he went to school with Madeleine’s brothers and described the family as ‘‘awesome’’.

‘‘They are an incredibly nice family.’’

Holliday said he was ‘‘incredibly shocked’’ when he heard that Madeleine was the girl caught up in the bomb scare.

‘‘I was so angry - that bastard’s got to go down.’’

A Scottish tourist, Ali Wilson, 23, said: "It's quite frightening.

"You don't expect to see something like this on your front door," she said, adding that she had just moved in to the street.

Last night, Ms Pulver's friends waited for updates. About a dozen of them embraced when she was released.

A neighbour who did not wish to be named said his family was asked to leave their home.

"My wife was in there and she was evacuated,'' he said. "They were just told to leave the area, so she got in the car and [went] off."

Another neighbour said she had guests for lunch at the time. "We saw a mass of police screaming down the road and coming in," she said.

"Police rescue and fire engines, and policemen on motorbikes - just masses of police."

The Pulvers

Mr Pulver is the chief executive of Appen Butler Hill, which provides linguistic and voice-recognition services for governent agencies and a raft of corporate clients, including Microsoft, IBM, Fujitsu, Hitachi, Toshiba and Motorola.

Born in Scone, Mr Pulver also worked for ACNielsen.

The Pulvers have three sons, who attend Sydney Church of England Grammar School (Shore), as well as their daughter.